Carrier Sense Multiple Access (CSMA) is often used in communications and is a typical probabilistic Media Access Control (MAC) protocol where nodes verify the absence of other traffic before transmitting on a shared physical medium, for example, a wired network or a radio frequency (RF) spectrum. The term “carrier sense” refers to a node's listening on a communications medium for a carrier wave or other distinctive feature of a transmitted signal in order to detect the presence of a prior transmission from another node on the shared medium, before attempting to transmit a signal on the same shared medium. If a prior transmission is detected, the node waits for the prior transmission to finish before initiating its own transmission. The term “multiple access” refers to the multiple nodes that send and receive on the transmission medium. Transmissions by one node can generally be received by other nodes using the medium.
In p-persistent CSMA, the letter “p” refers to the probability that a node having communications traffic to send will start transmitting in a specific period of time following the end of a received prior transmission. This is also referred to as the transmission probability, with values ranging from 0 to 1. A system in which a node having traffic to send always starts transmitting immediately once the prior transmission ends is an instance of 1-persistent CSMA, indicating there is a 100% chance that an immediate transmission will take place when a channel becomes idle. Waiting a random time before transmitting represents p-persistent CSMA, which is intended to reduce the probability of transmission collisions by giving different nodes different times at which they are permitted to start transmitting based on the transmission probability “p”. Each node with traffic to send waits a random or pseudo-random time before starting to transmit. The statistical distribution of the wait times is determined by the value of the transmission probability. As each node waits, it monitors the channel. If it detects the start of another node's transmission before its own transmission time arrives, it cancels or reschedules its own transmission so as to prevent the collision of multiple transmissions on the shared medium that would otherwise occur.
Some wireless digital communications media, for example high frequency (HF) communications systems require substantial Forward Error Correction (FEC) coding and interleaving to provide adequate digital voice communications reliability. Use of these encoding and interleaving techniques results in significant end-to-end delivery latency, which creates a severe vulnerability to transmission collisions, greatly reducing network traffic capacity in single-frequency networks. Many users rely on single-frequency communications because of their all-informed character, which means that any network member can hear any other network member's transmissions. However, single-frequency communications networks have been found to suffer from severely limited capacity due to frequent traffic collisions when their delivery latencies are relatively large.
In packetized digital voice communications, a voice signal to be communicated to one or more recipients is first converted into a sequence of digital data whose length is determined by the time duration of the signal. The digital data sequence is then divided into data packets of up to a fixed maximum length determined by the digital data network by which the data packets are to be delivered. For instance, in Voice Over IP (VoIP), the maximum sizes of the IP packets in which the digital voice data are contained is determined by the Maximum Transmission Unit (MTU) sizes of the data network and its component subnetworks. The division of the digital data sequence into packets adds significant overhead, but this overhead can be tolerated in these systems because the communications bandwidth (channel capacity) of the digital data network is much larger than the data rate of the digital voice data. The underlying digital data networks used to deliver the digital voice data in packetized digital voice communications use a wide variety of communications techniques, including some forms of p-persistent CSMA.
In non-packetized digital voice communications, the digital data sequence representing the voice signal is transmitted over the physical communications medium as a single unbroken sequence of modulated digital data instead of being broken into multiple packets. This is typically necessary because the communications channel capacity is not significantly greater than the digital voice data rate, so that the additional overhead that would result from dividing the digital voice data into multiple packets cannot be tolerated. Military and public safety radio systems frequently employ non-packetized digital voice communications because of the limited communications bandwidths available to these systems.
Designers of non-packetized digital voice communications systems are aware of the desirability of limiting end-to-end latency. In doing so, however, many system designers are motivated by a concern about system responsiveness and user acceptance, and not about network capacity. Some systems, such as the APCO Project 25 Land Mobile Radio system, attempt to detect incoming voice signaling as early as possible, to avoid potential collisions. It has been found, however, that network capacity achievable in this way is limited to approximately 54%, as in 1-persistent CSMA. Any transmission overhead reduces the effective network capacity to below 54%.